Thermosettable resins, for example, epoxies, are known to be useful in conjunction with fibrous reinforcing materials to form composite articles characterized by high strength to weight ratios. Such articles have found wide use in the aircraft and aerospace industries, as well as in other commercial applications where attributes such as strength, corrosion resistance and light weight are desirable. Fiber-resin materials have also been employed successfully in sporting equipment, such as tennis rackets and golf clubs.
Descriptions of various fiber-resin compositions are found in the patent literature. For instance, Frigstad in U.S. Pat. No. 3,472,730 shows filament-reinforced sheeting comprised of a heat curable epoxy resin composition. Russell in U.S. Pat. No. 4,178,406 describes fiberglass-reinforced composite articles of a cured binder resin and a method for their production. Fiber-reinforced epoxy resin articles useful as plastic piping material are disclosed by Johnson, et al. in U.S. Pat. No. 4,343,843. Prepregs comprised of a heat curable epoxy resin and reinforcing fibers, e.g., carbon, glass, aromatic polyamides, are described in U.S. Pat. No 4,309,473 (Minamisawa, et al.).
To improve the toughhess of fiber-resin matrix composites, it has heretofore been proposed to provide the fibers with a coating of elastomeric material. In one procedure, described in German Offen. No. 2,326,035, carbon fiber-epoxy resin composites are prepared by coating the fiber with an elastomeric polyester epoxide, e.g., adipic acid-bisphenol-A-diepoxide-butanediol copolymer, followed by embedding in an epoxide resin matrix. This procedure provides a plasticized epoxy coating on the fiber which is compatible with the matrix resin and can migrate away from the interface during the curing of the composite.
Tryson and Kardos describe a matrix compatible plasticized epoxy composition using Versamid-140 (General Mills) with glass monofilaments and rovings (36th Annual Conf. Reinforced Plastics/Composites Institute, The Society of the Plastics Industry, Inc., Feb. 16-20, 1981). Ying describes a non-reactive silicone rubber coating and an impact modified epoxy compatible finish (S.A.M.P.E. Quarterly, April 1983, pp. 26-32). The first material provides improvement in impact resistance but poor composite properties. The second material, being compatible with epoxy matrix resin, is expected to diffuse away from the fiber-matrix interface.
Enever, U.S. Pat. No. 3,943,090 (See also U.K. Pat. No. 1,358,276) describes a method of forming a composite of carbon fibers and synthetic plastics by incorporating the fibers in a matrix of the plastic. An elastomeric material, such as a urethane rubber, is provided in the matrix resin or, by surface treatment, on the fibers themselves to minimize the tendency for cracks to initiate at the fiber-resin matrix interface. The rubber is described as being compatible with the matrix resin, that is, it tends to move from the fiber surface into the resin matrix.
Thus, in all of the examples cited, either a compatible or a non-reactive coating is used. In the first case, the coating, being compatible with the matrix resin, is removed from the fiber interface by dissolution. In the second case, the coating creates a poorly bonded, weak interface which sacrifices internal strength properties.
Subramanious et al (Poly. Eng. Sci., Vol. 18, No. 7, 1978) have used electropolymerization and electrodeposition techniques to coat carbon fibers with polar and water soluble polymers. However, the resulting interface is susceptible to moisture intrusion and, therefore, it is expected--and as the work of Kaelble (J. Adhesion 8, 121 (1976) shows--that hot-wet properties of the composite are very poor.
Gynn et al (AFWAL-TR-81-4076) used polymers such as employed by Subramanious, for example, styrene/maleic anhydride, and later imidized the coating to render it hydrophobic. The coating in this case was a rigid layer, and no improvement in the impact strength of the composites was observed.
In the context of another kind of use, carbon fibers are described in Japan 75/58,396 as being impregnated with a rubber latex or liquid rubber, e.g., nitrile rubber, for improved performance in textile fabrics. Brook in U.S. Pat. No. 4,264,655, on the other hand, describes rovings such as fibrous strands, yarns, threads and tapes impregnated with a thermosetting resin such as an epoxy alone or in admixture with a reactive elastomer, e.g., carboxy-modified acrylonitrilebutadiene rubber, then overcoated with a nylon resin or other thermoplastic. In this case, all of the matrix resin, alone or plasticized with rubber, is applied to the fibers and then overcoated, e.g., with nylon; the overcoating is said to provide improved properties for textile operations.